Delayed viscous waterflooding



@fisicas ses v 3,208,518 f A DELAYED- viscous WATERFLoomNG John T. Patton, Tulsa, 0ld'a., assignor to Jersey Production Research Company, a corporation of Delaware Filed July 31,' 1961, Ser. No. 128,005

11 Claims.. (Cl.` 166-9) The present invention relates-to waterflooding processes usefulfor therecovery of crude oil from subsurface ref servoir'sand more particularly relates to a delayed viscous waterilooding process wherein an ionicv poiymer solution of low. viscosityis injected into an oil bearing reservoir i under controlled conditions such that an increase in viscosityswill take place as the solution displaces crude 'oil contained in the reservoir.

processes developed for the' recovery of crude oil from subsurface oil-bearing vreservoirs -in recent years. =Labora' tory work and field te'sts-have .shownlthat the injection of water -or-brine containing athickening agent in a concentration suicient to give a viscosity approaching thatof crude oil lwill generally permit the .displacement of subatent 3,208,5l8 Patented Sept. 28, 1965 i pH levels, and again exhibit'low viscosities at very high pH values. By adjustiing the pH of a-polymer solution which has the desired viscosity under conditions existing in the reservoir to be waterooded, a reduction in viscosity suicient to permit injection ofthe solution'into the reservoir at la reasonable rate withoutthe use of high injection pressures can readily be obtained; '.1 As the solution so. injected progresses into the reservoir from the injec- 'I vl5 Viscous waterflooding is lamong the more promising` the solution will increase simultaneously until the desired viscosity has been attained. Since injection rate is determined primarily by the rate of flow through the formation in theimmediate vicinity of the injection well, the delayed increasein viscosity which thus takes place in the process of'the invention permits the highv oil recovery `no rmallyassociated with viscous waterooding without an equivalentreduction in flood water injectivity.

The' 'figure of the .drawing illustrates theieffect of pH on viscosity of variousaqueous polymer solutions.

a A variety of water-soluble polymers of the ionic'type stantially moreoil from areservoir than could be obtained by wateriiooding the reservoir in the vusual manner. vOne difficulty frequently encountered in viscous waterflooding operations, however, is that of low'injectivity. The rate at which a fiuidican be' forced into a porous medium varies inversely .with viscosity and hence an increase in" .oodwater viscosity to obtain greater oil .recovery is acc ompariiedby a decrease in the rate-at which the solution l can be injected into the reservoir under a given pressure gradient.A This either necessitates a corresponding increase in the timeiallo'tted for completion of the wateriiooding operationv or-else requires'that injection pressures in cxcess ofthose-normally used in waterooding operations be employed. Neither alternative is attractive from an economics standpoint. Longer recovery periods tie up 4invested capital which might be madeavailable for other may be utilizedv as thickening agents in carrying out the process of-the invention. Such polymers characteristically have long spiral molecules. At intermediate pH values the molecules areloosely coiled and retard movement of the molecules of the solvent in which thev polymer is dis- A. solvedr Thisaccounts for the viscosity of such solutions. At pI- I values .outside this intermediate range, the polymer molecnle'scoil more tightly and hence the viscosity of the l solution is reduced. Since. this phenomenon is reversible,

the viscosities of aqueous solutions containing such polymers .can be varied at will over wide ranges by merely be of either the natural or synthetic type.v Suitable syn projects, result in'greater expense kfor the operationand f Y maintenance-of equipment, and reduce overall return on investment. -The 'useof higher injection pressures requires larger pumps thanA4 might otherwise be used, frequently necessitates the installation of heavier well bore casing and other new equipment in existing wells, and may pose problems due to inadvertent fracturing of the formation. 'Efforts to avoidthese difficulties have been largely unsucessful in the past and hence viscous watertlooding processes have yet lto be widely used in the petroleum industry.

. It is therefore an object of the present invention to pro-4 z .vide an improved watertlooding process which will permit I. an increase in tloodwater viscosity without an equivalent reduction in injectivity. A further object is to provide an Aimproved viscous watertlooding process wherein a substantial reduction in injectivity due to the high flow resistv,ance .in the immediate vicinity of the injection well is avoided'b'y introducing asolution of low viscosity into the reservoir under conditions such that a delayed increase in -viscosity will take place after the solution has progressed some distance into the reservoir from the injection well. Other objects willV become. apparent as the process of the invention is described in greater detail hereafter.

f In accordance with the invention, it has now been found that injectivity diliicultiesencountered with viscous waterooding processes utilized in the past can largely be avoided by employingl solutions of high molecular weight ionic polymers under controlled pH conditions. Studies have shown that aqueous solutions containing vwater-solu ble polymers of' the ionic type undergo. characteristic changes in viscosity with changes in hydrogen ion concentration. Such solutions normally have low viscosities at low pH values, reach maximum viscosities atintermediate changing the hydrogen ion concentrations.'

The polymers utilized for purposes of the invention may thetic polymers having ionic structures includes homopolymers prepared from acyclic unsaturated carboxylic -acids such-.as acrylic acid and substituted acrylic acids;

methacrylic acid with unsaturateddibasic acids and acid anhydrides such as fumarie acid, maleic acid and maleic anhydride;' copo'lymers of unsaturated dibasic acids and acid anhydrides-such as maleic acid and maleic'anhydride with vinyl aromatic compounds such as styrene, vinyl toluene, vinyl naphthalene, p-chlorostyrene, dichlorosty-` rene, trans-stilbene, isoallylbenzene, vinyl carbazol and .vinyl ferrocene; copolym'esof unsaturated dibasic acids and acid anhydrides s uch as maleic acid and maleic anhydride with unsaturated salts, esters and ethersA of mono and dicarboxylic acids such as methyl methacrylate, ethyl acrylate, allyl propionate, ethyl fumarate, isopropyl maleate, vinylmethyl ether, vinyl acetate, vinyl chloride and acrylonitrile; copolymers of unsaturated monocarboxylic acids such as acrylic and methacrylic acids with vinyl aromatics such as styrene and vinyl toluene; sulfonated polyvinyl aromatics such as sulfonated polystyrene, sulfonated polyvinyl toluene and sulfonated copolymers of styrene and vinyl toluene; and the like. Methods for the polymerization of organic monomers to produce these and simii.

v 3 obtained as plant-exudates, certain polysaccharides produced by the' fermentation of sugars, starches and other carbohydrates by bacteria of the genus Xanthomonasand similar organisms, andv ionic polysaccharides produced extracellularly by the yeasts -Cryptococcus, Laurentii var.

Flavescens NRRL-Y-l40l and the like. Other ionic water-soluble` polymers-will vsuggest themselves to those skilledinthe art. j l It jwill be' recognized that the solubility properties, stability characteristics, and -thickening efficiencies of water- 10.

soluble ionic polymers differ and thatall such polymers are therefore notequally el'ective for purposes of the invention. Ingeneral vthe ionic polysaccharides have high thickening eiliciency, are highly stable under reservoir conditions, and can be produced at lower cost than can thes'ynthetic polymers, The use of these materials, parl ticularlyl thoseproduced by Xunthomonas campeszrs,

Xanthomonasbego'ni,*Xamlzomonas plzaseol, Xantliomoms hederae, Xanthomvnas incanae, Xanthomonas carotqe, Xanthomonus rranslcens and related organisms,

is therefore preferred for purposes of the invention.

Theionic Water-'soluble polymers employed as thickening agents will normally have molecular weights inthe range between .about 300,000fand' about 5,000,000 Staudthe caseof a'particular ionic polymer lwill depend upon thel polymer structure and the solubilizing groupsl present on `the molecule?. Inffmost'cases molecular weights in excess of about 1,000,000 Staudinger are preferable.

in carrying out the process of the invention, an'aqueous 30 ionic polymer solution having suiicient viscosity under normal reservoir conditions to permit eiective displace- 'l ment of Acrude oil from the reservoir to be waterilooded whereil( designates the reservoir permeability, Vrepre sents viscosity, and the subscripts w and o denote water and oil respectively.l A mobilityratio ofunity indicates that the water and oil will move through the reservoir in the presence ofone another -.with equal ease.l Ata mobility ratio greater than 1, displacement of oil by the .watervis relatively ineiiicientand hence the water tends -to bypass oil contained in zones of low permeability. A 50 :mobility vratio less than 1l permits reasonably effective displacement of the oil by the injected wateizM--Wherecpractical, the viscosity of the aqueous solution should be suicient togive a mobility ratio less than-l.. It will be understood, however, that process efficiency can be improved by increasing the viscosity of the water even though the increase is not vsuilici'ent to produce a mobility 'A ratio less than l.

The ionic polymer concentration required to 'give the Y desired Yviscosity will depend upon the particular polymer 'l utilized.' In general, the concentration necessary depends primarily upon the molecular weight of the polymer.

'The synthetic polymers are normally used in concentrations between about 0.05 percent and about 2.0 percent lby weight. The polysaccharides generally have higher molecular weights than the synthetic compounds and are therefore effective in concentrations between about 0.001 percent and about 1.0 percent by weight. The concentration required inthe case of a particular polymer can readily be determined in the laboratory.

' Following preparation of the viscous polymer solution to begutilized, the viscosity is reduced by adjusting the pH of the solution. The necessary reduction can generally i' vbe obtained by either increasing or decreasing the pH level. The method employed will depend upon the par- The molecular weight' range most eiective in 25 ticular polymer utilized and the: characteristics of the reservoir to be waterooded. Solutions containing certain polymers undergo greater reductions in viscosity at low pH values than at correspondingly high pH' levels and hence the addition of an acid or a salt of -a strong acid and a weak base to depress the pH will be, preferable. Suitable acids and salts include hydrochloric acid, sulfuric acid, nitric acid, acetic acid,- formc acid, propionie acid,

maleic acid, succinic acid, aluminum chloride, magnesium chloride, manganese sulfate,- zinc nitrate and the-v like. In other cases the addition of a base orasalt of aweak acid and a strong base is preferable. Bases and salts which may be employed include ammonium hydroxide,

odpim hydroxide, potassium hydroxide, sodium acetate,

sodium carbonate, potassium tartrate and the like.j The use of an alkaline reagent may bepreferred in carbonateV t the desired reduction in viscosity, it may be pumped into l' the reservoir ;through.one or more injection wells in the conventional manner. Asthe injected solution moves aw-ayfrom the'injection well, the pH gradually changes due to theneutralization of acidic'or basic constituents in the solution by materials present in' the reservoir. In the case'of an acidic solution. for example, excess hydrogen ions may exchangewithsodium inv the clays present in the reservoir to effect a reduction' in pI-l; while hydroxyl ions in abasic solution may reactwithhydrogen ions and other substituents-from the reservoir.y ."lhis neutralization, accompanied by an increase of the- 'viscosity of the injected solution, continues. until the pH of the solution reaches a-valueequivalent to that of the reservoir.

The injected aqueous solution thusdoes notreach the vfull viscosity needed for effectivey displacement ofthe crude oil in the jre'servoiruntil'it'ha's advanced some i distance -from the injection well'. The process ofv the i invention istherefore referred to as' a. delayed viscous waterflooding process. v, I,

.Th`e delayed increase in viscosity within the 'reservoir UAdoes nothave a significant adverse etfectuponinj'ectivity.

Resistance to flow of the injected solution through-the reservoir rock is highest immediately adjacent the injection well because of the relativelysmall reservoir cross-section s Athrough whichjthe 'solution must move.i "The1cross-sectional areal rapidly increases as: the solutionjmoves outwardly from the well bore and hencetheresistance to ow quickly becomes very small. At a distance .20 feetifrom the well bore 'for example, the pressure .'-g'radientv required to move the tluid'is only about 1/40 of that 'necessary to move it an incremental distance from the well bore into the formation. The effect of the delayed-increase in viscosity within the reservoir on the pressure required at the injectionv well is therefore quite small. As a result, the process of the invention obviates the necessity forv injection pressures significantly in excess of those normally employed forinjection purposes and for a given pressure gradient. permits more rapid injection of the'iioodv water than might' otherwise be obtained in a viscous waterooding operation.

Water or brine containing an ionic polymer may be injected into the' reservoir in the above manner throughout the waterooding operation. Satisfactory results can generally be obtained, however, by adding the polymer to only a portion of the water or brine injected into the reservoir. This leads to the establishment of a bank of 'viscous solution in the reservoir surrounding the well used for injection purposes. Ordinary water or brine containing no` thickener can then be injected in order to propel the bank of viscous solution through the reservoir toward one or more production wells from which the displaced crude oil is to be recovered. The volume of thickened water used inan operation of this type should,

of course,'be suicient-to prevent the later injected water.

from breaking through the bank and contacting the reser ',voir in advance of'theviscous solution. -A diminishing concentration of polymer beginning at the leading edge of the bank and continuing to the trailing edge is generally preferable in order to avoid a premature breakthrough of v thevlater-injectedwater.- The required thickness of the bank used will-depend upon the distance over which it is to be propelled, the nature of the porous rock in the reservoir, andthe viscosityv of the liuid in the bank. In

'an operation carried out with a single injection well and one or more. production wells spaced a distance of 600 vfeet or more. from the injection well, a bank of viscous solution about 100 to about 400 feet thick should ordinarily be establishedin the vicinity of the injection well,

to' assume persistance of the bank during the entire wateriio'oding operation; The volume ofviscous solution which must be injected to establish such a bank can readilyrbe vcalculated from information as to the distance between -the injection well and the most remote production well,

acid-or base is normally sufficient to obtain the delayed increase in viscosity.

The process of the invention is further illustrated by the 1 following examples.

EXAMPLE I A copolymer prepared by the polymerization of acrylic -acid and styrene in a benzene diluent with an azobis isobutyronitrile catalyst was dissolved in distilled water in .a concentration of 0.2 percent by weight. The polymer utilized had a molecular weight of about 500,000

Staudinger. The pH of this' solution, measured elec-v trometrically, was found to be 6.38. Measurement of the viscosity at this pH level with a Brookfield viscometer at 6 revolutions per Vminute gave a value of- 12.2 centipoises. The pH of the solution was then increased stepwise by the addition of dilute sodium hydroxide and the viscosity was-determined at'various pH levels. The relsults obtained are-set forth in the following table and the average thickness of the reservoir, and the average porosity of the reservoir. Such information is normally available with respect to any reservoir in which a water flooding operation is to be carried out. Asa general lying the lateral area defined bythe flood pattern.- In a illustrated in the accompanying drawing.

'I I 'able I FFECT'OF CHANGES IN pH N VISCOSAITY OF ACRYLIC A CID-STYRENE COPOLYMEB. SOLUTION large reservoir, several -ood patterns may be carried out simultaneously. The quality otviscous solution employed ineachpattern 'should be based upon the volume of the reservoir. underlying that. pattern.

ooding operations for example, it is normally assumed t that the pore volume of the reservoir under ood between each row. of injection wells and an adjacent row of pro- I ductionvwells is the total pore volume of the reservoir underlyingthe lateral area between the two rows. The

In line drive water'- quantity ot' water or brine containing the ionic polymer to be injected through a row of injection wells toward af row'of producing wells should then be from lto aboutA 50 percent of the total reservoir pore volume underlying the area betweenthe two rows.

- The ionic polymers employed as thickeningl agents in the proces of the invention may be utilized in conjunction with other materials conventionally used in waterooding operations. Such materials include bactericides and preservatives, surface active agents, corrosion inhibitors, and tracers designed 'to permit a check upon the progress of the liiooding operation.y Many suitable materials which may thus be employed will be familiar to those skilled in the art. Care should of course be taken to ascertain that the materialsselected'do not adversely atect the ionic polymers employed in the procesor prevent control of the pH of the polymer solutions.

pH Viscosity,- pH l Viscosity,

l Centipoises Centipoises Itcan be seen from the data in Table Iabove and from curve A inthe drawing that the solution containing the acrylic acid-styrene copolymer increased in viscosity at .pH values between about 6 and about 9 and decreased in viscosity as the pH level was changed from about 9 to about l2.- This behavior is typical of ionic water-soluble polymers. Byinjecting an acrylic acid-'styrene copolymer -solution having'a pH below about 6 or above about 12 into areservoir having a normal pHibetween about 7 and about ll, an increase in viscosity from l4lcentipoises or lower up to about 25 centipoises can be obtained without 'the disadvantages-.which would normally accompany the injection.- of :1,25 centipoise solution into the reservoir. Since the concentration of the polymer in the solution determines the viscosity'at a particular pH level, the initial and final viscosities can readily be controlled by use of the polymer in the proper concentration.

, EXAMPLE n A copolymer of vinylmethyi ether and maleic anhydride n produced by the low temperaturel reaction of the mono- In a modification ot the process of the invention, an Y acidic or basic solution maybe injected into the reservoir tobe waterflooded prior to the introduction of the ionic A'polymer solution.v Neutralization of the polymer solution will generally occur in response to reaction of the acidic or basic constituents in the polymer solution with materials native to the reservoir.

In certain sandstones and somev other formations, however, the injection of an acid or basic solution to permit better control of the f neutralization may be desirable. Such a solution will generally be followed by a slug of untreated water or brine to prevent a premature increase in the viscosity of the later injected polymer solution. A small bank of dilute 1v-F* w n i o s; s

mers in the presence of an azo-bis-isobutyronitrile catalyst was added to dstilledwater in a concentration of 0.5 percent by weight. The viscosity of this solution was measured with the Brookfield viscometer at pH valu'es between 3.6 and 11.2. The results are shown in Table II below.

Table Il EFFECT 0F CHANGES iN nu oN vrscosr'rY oF VINYL- METHYL ETHER-MALEIC ANHYDRIDE COPOLYMER SOLUTION pH Viscosity, pH

Viscosity, C eutl poises Centpoises The data in Table II and curve B in the drawing further demonstrate the effect of changes in pH upon the viscosity of aqueous solutions containing ionic polymers. It will be noted that the vinylmethyl ether-maleic anhydride copolymer solution had a relatively low viscosity at. pH

' Statues or 3.6 and 11.2 but centipoises at the neutralpoint. The injection of such a l ooding operations.

had a 4'viscosity of nearly 45 solution at `a pH less than about 3.6 orvgreaterthan A.

about 1.1.2 into a reservoir having a normal pH of about 7 would permit a significant' delayedincrease in viscosity as'the pH of the solution approached that4 of the reservoir.

Again the initial and final viscosity values could'be increased or decreased b y changing the polymer conceny' tration.

' EXAMPLE 1n j A high molecular weight-polyacrylic acid was dissolved' Y in distilled water and testedin the manner vdescribedabove. Theetect of changes in pH upon the :viscosity of. the solution is shown-.by the' data in Table III below Again it will be noted that-.the polymer solution exhibited a high viscosity over a broad intermediate range in pH values and had much lower viscosities outside that range. This typical behavior of dilute aqueous-solutions thickenedV with ionic polymers permits the use of suchpolymers to obtain a delayed increase in'viscosityv during viscous water- EXAMPLE IV y vcommercial water-soluble ionic polymer of the methl acrylic type marketed as a thickener for aqueous media was' added to distilled water in a concentration of ).2 percent byweight. The viscosity of this solution was then determined at several pH levels in the manner described earlier. The results obtained are shown in Table IV and by curve .D in the drawing.

Table 1V EFFECT OF CHANGES IN pH ON VISCOSITY OF METH- .ACRYLIC POLYMER SOLUTION 'pH Viscosity, pH Viscosity,

Centipoises Ceutipoises 1. 5 81.2 2. Z X0. 5. T6. 0 35. 2 52. 5 (i9, -5

The data in Table 1V above show that the viscosity of l the solution underwent a change from about 1.5 centipoises to about 81 centipoises as the pH was increased from about 3 to about 8 by the addition 'of sodium hydroxide. This change in viscosity permits the injection of a polymer solution into a subsurface reservoir to form a highly viscous bank of solution without the injectivity problems which -would otherwise be encountered.

EXAMPLE V A water-soluble ionic polysaccharide was produced by 'inoculating a sterlie solution containing about 2.0 weight percent of raw sugar, about 0.1 weight percent of dipotassium acid phosphate and about 0.05 Weight percent .weight percent of the polysaccharide. This solution had N at the 12.4pH level.

the following table and illustrated by curve' E in the drawing. s

andby curve C in the drawing.- j

, Tamm y EFFECT OF CHANGES IN pH'ON VISCOSITY OF POLY-- v ACRYLIC'ACID SOLUTION l s pH Viscosity, pl'I Viscosity,

.' Centipoises Centipoises v 2.s9. Q 'M7-.9 59.5 22.7 56.4 29.9 36.0 41.2 18.9 50.5

. wise untila pH of 2.10 l had been attained. Viscosity measurements were made with the Brookfietd'v'iscometer as vthe pH decreasedx- The pH of the solution was then raised lto 'a .nal value of 12.4 by the addition of dilute sodium hydroxideandlviscosity measurements were again made at various-pH values. I tfwas .found that the viscosity reached la maximum of about 31.4 centipoises ata pH.

of about 10.2 and then declined to" about 16.0 centipoises These results are summarized in fable V FECT OF CHANGES IN pli ON VISCOYSITY 0F SOLUTION CONTAINING POLYSACClIARIDE PRODUCED BYXAN- TIIOMONAS CAMPILSTRIS pH Viscosity, pH Viscosity, y. Ceutipoises Centipoiscs The results set forth in Table-V and depicted by curve' E in the drawing demonstrate that the eect of pH on the viscosity-ot aqueousl solutions containing water-soluble' ionic polymers is not'limited to `'solutionsprepared with synthetic polymers. A variety ,of polysaccharides and similar polymers derived from natural sources exhibit pH- viscosity'relationships similar to thoseof the synthetic polymers and can therefore Abe utilized for purposes ofthe invention.

' l p l EXAMPLE VI.v The process ofthe invention can be better understood by considering a delayed viscous .waterflooding operation designed to recover crude oil from a sandstone reservoir containing about 3 percent clay. vThe reservoir-has a relative permeability to water about 0.2 times the relative permeability to crude oil and is therefore classified as a slightly water-wet reservoir.v The absolute permeability is about 300 millidarcies and the porosity is about 24 percent. Oil saturation -at the onset of the waterooding operation is about 45 percent; while the water saturation is. about 30 percent; The viscosity of the oil in place is `about 20' centipoises. -Abou't 1200 acres ofthe reservoir are to be waterooded. The average sand'thickness is about 30 feet. The vreservoir thus contains about 30 million barrels of oil in place.

Calculation of the oodwaten viscosityi'required to -about 6 centipoises at the reservoir pH of about 8 when present in dilute aqueous solution in a. concentrationl of about 0.02 percent by weight.

In carrying out the Ywateroorling operation, a viscous polysaccharide solution is first prepared in batches by adding the polysaccharide to brine obtained from producing 'wells in. an adjacent eld. The solution is acidified by the reservoir through. injection wells located in multiple vespot patterns. Injection of the low viscosity. ionic polymer solution is continued-in th'is manner until a total of about *3 million barrels have-been introduced'into the reservoir.

At .this point a bankfof the solution has been established 'i about each of-'theinjection wellsin cachot the live-spot patterns. The-:recovery `operation is` then continued by injecting ordinary brine containing no polymer into the reservoir through the 'injection wells.

-. As thel injected polysaccharide solution flows into the reservoir adjacent each of the injection wells, the hydro- `ch loric,acid contained in the solution reacts with carbonates and other alkaline materials in the clays.with

' reservoir.v As'the pH increases, a corresponding increase v in the-viscosity of the injected `solution takes place. At

"the reservoir pH of about 8, the solution has .a viscosityl of about 6 centpoises. The bank of viscous solution thus 1 l .established is moved outwardly from the vicinity of the injection wells by the later injected brine. The viscous bank promotes more effective displacement of the crude oil and reduces preferential flow of the brine through high permeability zonesin the reservoir?. 4Crude oil displaced vbythe solution flows into the production wells.

and is pumped to the surface in the conventional manner. The operation is` carried out until the oil-to-water ratio becomes. so low that further injection of brine is uneconomical. Much more total oil' is recovered from th'ev reservoir than-could have been obtained in a conventional `waterooding process carried out inthe absence cfa' thickening agent.. The delayed increase in viscositymade possible by the use of an ionic polymer under controlled i pHconditions signiiicantly improves the economics of` the v:'process by permitting higherinjection rates than could @otherwise :have been obtained atthe injection pressure utilized.v

A. J 'EXAMPLE VII i In secondar'y`-recovery operation for the Adisplacement of a 14 centipoise oil from a reservoir having a pH of about .7, a SOlutionof a sulfonated copolymer of styrene and ,-.vinyltoluene is employed. The sulfonated polymer solution havingA a-viscosity of 14.8 centipoises at a pH voitS. Hydrochloricacid is added to the polymerv solu.

tion until the pH-i's decreased to a value of about 2.1.

I The relationship between pH and, viscosity forthispartisular polymer over, the pHrange! between about 2.1 vand 9.1 is shown inthe following, table, the values for which were obtained by titrating. a 0.3 weight percent ysolution'with hydrochloric'acid and periodicallyv determining pH and viscosity values in the manner described inExamples 1 throughS.

. Table VI .EFFECT oF CHANGES '1N pn oN vrscosr'rY oF sULFo` NaTED COPOLYMER SOLUTION .-pH` Viscosity, pH Viscosity,

Ceutipoises Centiposes The reservoir to be flooded with the polymer solution i is pretreated by injecting a 1 percent solution of sodium hydroxide into the reservoir through each of the injection wells. The quantity of NaOH injected at each well is determined by calculating the amount required to reduce the'pH of the later-injected polymer solution to the dewhich it comes into' contact. The pH o f the solution gradually approaches4 that of the 4connate waterfinithe sired level by reacting with acidic constituents in the solution. Following this introduction of sodium'hydroxide.

solution into the reservoir, brine isinjected at each well to prevent premature reaction of acidicconstituents in `the polymer solution. The solution containing the sulfonated copolymerof styrene and vinyl toluene is then injectedinto the reservoir through each-.of the injection '.well-s under a pressureof about '500 pounds per square inch, only slightly in excessof the injection pressure which would normally beused in a conventional water? ooding operation in this reservoir. After a volume of y polymer solution equivalent to about 10 percent of the reservoir pore volume has been injected, the injection of brine into the reservoir is commenced. Injection of the brine is'continued until a high oil-to-oil ratio at the production wellsvindicates that the economic limit of the waterflooding operation has been reached.

.The sodium hydroxide solution'employed to pretreat the reservoir results in a high sodium lhydroxide concentration on the rock surfaces with which the later-injected polymer solution comes into Contact. The acidic constituents-of the polymer solution are neutralized by reaction with the sodium hydroxide and hence the viscosity lof thevsolution increases from the initial value of about 2.3 centipoises to a vicosity of about 15 centipoises. The delayedincrease in viscosity thus obtained in accordance [with the invention permits'injectionof the polymer solution into the reservoir at pressures commensurate with those employed in conventional wateriiooding operations 30 without a sacrice 'in injection rate; The bank of viscous solution permitsfmore effective 'displacement of crude oil toward the production wells than could be obtained in a( conventional waterooding operation carried out without j a thickening agent.`

It vwill .beevident from the foregoingthat the delayed viscous waterooding process of the invention may be carried outl with a wide variety of water-soluble ionic polymers and is not limited to the use of the specic polymers referred to in thel preceding examples. The delayed increase in'viscosity obtained with 'such polymers under controlled pH conditions may be utilized in a variety of operations vrequiring the establishment of a bank of viscous duid within a subsurface formation.

What is claimed is:

l. A process for recovering crude oil from a subsurface oil-bearing reservoir which comprises preparing an aqueous solution of a high molecular weight ionic polymer,

- said solution having a viscosity at the pH of said reservoir in excess of that of water; adjusting the pH of said solution to obtain a viscosity below that of said solution at -the pH of said reservoir; injecting said solution into said reservoir at the reduced viscosity through at least one injection well penetrating said reservoir; and recovering crude oil displaced by said solution through at least one production well penetrating said reservoir.

2. A process as defined by claim 1 wherein said solution is an aqueous solution of an ionic polysaccharide.

3. A process as defined by claim 1 wherein said solution is an aqueous solution of a sulfonated polyvinylaromatic.

4. A process as defined by claim l wherein the pH of said reservoir in the vicinity of said injection-well is adjusted, prior to the injection of said polymer solution, by injecting an aqueous solution containing a compound which will react with constituents of said polymer solution toetfect a change in the pH of said polymer solution.

5. A process for displacing crude oil from a subsurface oil-bearing reservoir which comprises dissolving a watersoluble ionic polysaccharide in water in a concentration between about 0.001 percent and about 1.0 percent by weight, adjusting the pH of the solution containing said polysaccharide to obtain a viscosity less than the viscosity of said solution at the pH of said reservoir, injecting said solution into said reservoir through at least one injection well penetrating said reservoir, and recovering crude oil Patent No. 3,208,518

A in the heading to. the printed specification,

displaced by said solution through at least one well penetrating said reservoir. 1

6. A process as defined by claim 5 wherein said polysaccharide is an ionic polysaccharide produced by the action of bacteria of the genus Xanthomonas on a carbohydrate..

7. A' process as defined-by claim 5 wherein the "pH of '5 said poiysaccharide solution is adjustedby theA addition of an acid to said solution. i 3. .A process as defined by vclaim 5 wherein.- the pH of" production 11. In an-oil recovery operation where a bank of visxcous aqueous solution is-injected into a subterranean formation surrounding an injection well to'displace .oil contained in said formation toward a'production well,

ularweight water-soluble ionic polymer and a base to polymer and acid into said formation., and thereafter .Y recovering oil displaced by said solution atsaid producl the improvement .which comprises adding a high molec lo an aqueous liquid in concentrations suicient to produce said reservoir'. in the -vicinity of said injection well is a solutionhaving a viscositybelow that of a solution'conadjusted, prior to the injection oliy said polysaccharide taining'said polymer in the same concentration at vthe pH solution, by injecting into said reservoir through saidof said formation, injecting said solution containing said injection wella bank of aqueous solution containing a polymer and base into said formation, and thereafter recompound which-will react with constituents in said poly- 15 -bialPolysaccharides, Apr-il 1 9'61,-3 pages '-cHARLEs-E. ocoNNELnfPrimary Examinar.'

saccharide solution to effect a change in the pH of said well. roiyscharide Solulrgrd *b s h d nomma-cited bythe Examiner Y processas ei 'e y c aim w erein sal aqueous solution is a sodium carbonate solution.' UNITED STATES PATENTS c 10min-an oil recovery 'operation wherein a bank of 20 25612485- 9/52 Baci et al- 252-85 .-viscous aqueoussolutionds injected into a subterranean" 2,950,760 8/60. Bernard -166-9 formation surrounding aninjection .well to displace-oil 3,000,790 9/61 Jeanes etal. 195-31 contained in` said formation toward a lproduction well, 3,007,379 11/61 Jordan --4--+-;. 260-209 Y the'irnprovernent which comprises adding a high molec- A ularweight water-soluble ionicpolymer and anfacid to 25 OTHER REFERENCES anlaqueous liquid in concentrations suflicien't to produce jU S Depfj of l Agriu1mfe, Pub1` CA .N 14 Micm- .a solution having a viscosity below that of a'solution containing said polymer'in the same concentration at the pH of `said formation`,injecting said solution containing said UNTED STTES PATENT FFICE caariricars or connection September 28, 1965 John T` Patton the above numbered pat- A 'i t a ears in It yis hereby certified tha II'OI PP S Patent Should read as ent requiring correction and. t.hat `the said Letter corrected below. i

i linesY li/tomS-,Y- for "assigner to Jersey a corporation of Delaware" read to Esso Production-Research In the grant, Pro-duction Research Company,

by mesne assignments,

assi nor, line Companyg, of Houston, Texas, a corporation of Dlaigessrs" vl2 for "Jersey Production Research Company, i s a its successors read- Esso Production Research Company lines 3 and 4, fof- Research Company,l a corporation y. rse Production assignor to Je Y by mesme as slgnments, to Esso nor of Delaware" read assig lon of Production Research Company, Houston, TeX., a corporat Delaware (SEAL) Attest:

Envir-.an HRFXSLR Fammi. c i me: Of" Ptit 6111115 covering oil displaced by said solution at said production 

1. A PROCESS FOR RECOVERING CRUDE OIL FROM A SUBSURFACE OIL-BEARING RESERVOIR WHICH COMPRISES PREPARING AN AQUEOUS SOLUTION OF A HIGH MOLECULAR WEIGHT IONIC POLYMER, SAID SOLUTION HAVING A VISCOSITY AT THE PH OF SAID RESERVOIR IN EXCESS OF THAT OF WATER; ADJUSTING THE PH OF SAID SOLUTION TO OBTAIN A VISCOSITY BELOW THAT OF SAID SOLUTION AT THE PH OF SAID RESERVOIR; INJECTING SAID SOLUTION INTO SAID RESERVOIR AT THE REDUCED VISCOSITY THROUGH AT LEAST ONE INJECTION WELL PENETRATING SAID RESERVOIR; AND RECOVERING CRUDE OIL DISPLACED BY SAID SOLUTION THROUGH AT LEAST ONE PRODUCTION WELL PENETRATING SAID RESERVOIR. 